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Old 01-14-2006, 09:59 AM
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Default Re: The C.A.S. and Digidesign Present

Quote:
when color television was being developed, engineers looked into how to make the new NTSC system backwards compatible with the old monochrome system. they thought by adding the color information on a sub-carrier, they could transmit both sugnals and still use the old radio transmitting technology. but this color signal would interfere with the audio subcarrier.
To elaborate on the interference...

The argument when color was developed was that the frequency of the color subcarrier would create beating with the sound subcarrier that would be visible on some black and white television sets. The sound carrier, however, is frequency modulated. Therefore, beating would have only occurred at a specific frequency. A GE engineer determined that if the frame rate was dropped by .1% (from 30 to 29.97), that the beating would be reduced, and compatibility would be maintained (Lehrman 220). As a result of this change, 60Hz AC cannot leak into a video signal, or “bars appear to roll through the picture every 17 seconds” (Schubin 29).

The equation that has driven audio and video engineers mad by creating this non-whole number for video sync is:
[(number of scanning lines per frame•frames per second)/2]•455=color subcarrier frequency (Schubin 29). When the appropriate numbers are inserted, it becomes:
[(525•29.97)/2]•455 --> (15,734.25/2)•455 --> 7,867.125•455 --> 3,579,542
The NTSC adopted this equation, and could not change the lines per frame (or all TV sets would be obsolete), so they changed the frame rate. The idea behind the number 455 is frequency interleaving of the video and color signals, which would minimize interference between brightness and color data. The number 455 produces a result that is an even number of half the line rate (Schubin 29).

Now who can tell me why CD-quality audio is sampled at 44.1kHz (hint - its related)?

Sources:

Lehrman, Paul D. “SMPTE-ed Off--Why We Can’t Drop Drop-Frame.” MIX
Aug. 2001: 20-22, 218-222.
Schubin, Mark. “A Page of History.” Videography Apr. 1993: 29.
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